1. Field of the Invention
The present invention relates to a silicon nitride circuit board in which a copper system (a copper and a copper alloy) circuit plate is bonded to a silicon nitride substrate by the use of the copper directly bonding method, and a manufacturing method therefor.
2. Description of the Related Art
Conventionally, there have been used various kinds of substrates such as a ceramics substrate, a resin substrate, and etc., as a substrate for mounting electronic components such as semiconductor devices, etc. Especially, many ceramic substrates have been used for the circuit board for mounting the electronic components having high radiation efficiency, since the ceramic substrate has satisfactory electrical insulation and heat radiation efficiency, and etc.
Also conventionally, the above-mentioned ceramic substrate has been mainly made of a sintered alumina. However, the alumina substrate has its limit with respect to the heat dissipation efficiency. This is because that the heat generated in the semiconductor device tends to increase year by year with the increases of the integration, the frequency and the output power of the semiconductor device realized in recent years. Therefore, there is put to practical use the ceramic substrate made of an aluminum nitride (AlN) which has about ten times the thermal conductivity of alumina and is approximate to silicon (Si) in coefficient of thermal expansion.
Since the aluminum nitride substrate has the above-mentioned characteristic, but has not high mechanical strength and toughness, it has disadvantages that features are generated due to the clamping executed at the assembling step, and that cracks are apt to be generated when the thermal cycles are applied thereto. Therefore, the aluminum nitride substrate is inferior in mechanical reliability among the ceramic substrates for mounting the electronic components, which depends on the element characteristic.
There is noticed the ceramic substrate made of a silicon nitride (Si.sub.3 N.sub.4) which is inferior in thermal conductivity compared with the aluminum nitride substrate, but is approximate in thermal expansion to silicon (Si) then is superior in mechanical strength and toughness. The silicon nitride substrate has been capable of obtaining the thermal conductivity not less than, for example, 40 W/m K, by controlling the particle size of the silicon nitride raw material, the composition of the sintering auxiliary composition, and etc.
In a case where the above-mentioned silicon nitride substrate is used as the circuit board, etc., it is indispensable to bond a metal circuit plate to a surface of the substrate similarly to the ordinary ceramic substrates. It is studied that, when the circuit plate is bonded to the silicon nitride substrate, a copper system circuit plate is employed as the circuit plate, and then this copper system circuit plate is bonded to the silicon nitride substrate by the use of the copper directly bonding method (hereinafter referred to as "the DBC method") or the metal activating method.
In a case where the copper system plate is bonded to the silicon nitride substrate by the use of the DBC method, an oxide layer is required to be formed on a surface of the silicon nitride substrate because a copper-oxygen system eutectic compound is used for the DBC method. The oxide layer is formed on the surface of the ceramic substrate usually by the use of the thermal oxidization method. However, there is a problem that the stable oxide layer which is required for the DBC method is hard to be formed on the silicon nitride by the use of the thermal oxidization method. This is because that the silicon nitride has complicated fine structure and that an Si--O system oxide (SO.sub.2, etc.) formed by the thermal oxidization method can be of various kinds of crystal structures.
That is, the oxide layer having a thickness of about 1 to 3 .mu.m is necessary for the purpose of the bonding of the copper system circuit board due to the DBC method. When the oxide layer having such a thickness is formed on the surface of the silicon nitride substrate by the use of the thermal oxidization method, fine cracks and pores of large diameter are inevitably generated. The liquid phase of the copper-oxygen system eutectic compound (Cu--Cu.sub.2 O eutectic compound, etc.) generated at a high temperature is utilized for the bonding by the DBC method, the liquefied eutectic compound penetrates through the oxide layer to the silicon nitride substrate if the cracks and pores exist in the oxide layer. The liquefied eutectic compound which has reached the silicon nitride substrate acts upon the silicon nitride to generate nitrogen gas, which gas causes a generation of the blister and results in a poor bonding strength between the silicon nitride substrate and the copper system circuit plate.
Further, Si--O system oxide such as SiO.sub.2, etc., has a disadvantage that the thermal conductivity thereof is very low. In a case where the copper system circuit plate is bonded to the silicon nitride substrate through the Si--O system oxide layer which is low in thermal conductivity, the Si--O system oxide layer causes to prevent the thermal transmission (radiation) from the semiconductor components, etc., to the silicon nitride substrate. Therefore, the thermal conductivity which is essentially possessed by the silicon nitride substrate cannot be utilized.
From the above, in the silicon nitride substrate applied by the DBC method, there are requested to be capable of easily forming the oxide layer which is stable and has a sufficient thickness, and then is indispensable when applying the DBC method to the silicon nitride substrate, and to depress the increasing of the thermal resistance due to the oxide layer.